This invention relates, in general, to improving the bondability of tape automated bonding (TAB) semiconductor package leads to circuit boards and more specifically to depositing tin over copper-tin intermetallic compounds that form on tin plated copper TAB leads during manufacturing, burn-in, and storage that reduce TAB lead bondability.
The increasing demand for economical high lead count semiconductors lead to the use of TAB (tape automated bonding) leads as an interconnecting mechanism between a semiconductor die and a circuit board. One of the TAB structures utilized tin plated copper as the TAB leads. Intermetallic compounds formed on these type TAB leads during manufacturing, burn-in, and storage of completed TAB semiconductor packages. The intermetallic compounds reduced the amount of tin on the lead's surface thereby reducing the reliability of TAB lead bonds to circuit boards and limited the package's usefulness. Bonding TAB leads that contain insufficient amounts of tin on the lead surface to a circuit board required the use of strong, difficult to remove bonding fluxes. These fluxes, and their removal after bonding, damaged the thin delicate TAB leads. Therefore, it is desirable to have sufficient amounts of tin on the leads to permit circuit board bonding with mild, easy to remove bonding fluxes and thereby minimize damage to the TAB leads.
During intermetallic compound formation, copper in the lead frame combined with the plated tin that would normally be available for bonding the leads to a circuit board. Before a reliable bond could occur, the intermetallic compound that had formed had to be covered by a new application of tin. Tin application techniques for TAB leads included both electrolytic tin deposition, and selective electrolytic plating of tin onto the TAB lead section to be attached to a circuit board.
Electrolytic re-plating of the TAB leads after TAB semiconductor package assembly required a TAB lead frame that was originally designed and manufactured with special conducting bars in the TAB lead frame. The conducting bars were required for plating current flow in the area to be plated. These re-plating bars shorted all leads together and prevented the packaged device from being tested until the bars were removed from the lead frame. Since the packaged device had to be tested at several manufacturing stages before it was plated, these plating bars complicated the manufacturing process and increased manufacturing costs. Additionally electrolytic re-plating was difficult to control on TAB leads after the package assembly process was complete. Complicating the lack of electrolytic re-plating control were the TAB lead shapes on a lead frame. Leads varied in length, number of bends, and angle of bends all of which added to the difficulty of controlling electrolytic re-plating current densities and associated plating thickness. Electrolytic re-plating limited the TAB lead frame application to one particular die, increased costs, and restricted testing.
When the TAB lead frame was manufactured, selectively plating a thick tin layer on the TAB lead portion to be attached to a circuit board would provide additional tin on the lead surface, and reduce the impact of intermetallic formation. However, the amount of tin required to maintain a surface layer of tin after intermetallic compound formation was determined by TAB storage time, in addition to the time and temperature of the TAB semiconductor package assembly process. Consequently it was difficult to determine, during lead frame manufacturing, the correct amount of tin required to completely eliminate the impact of intermetallics that subsequently formed. This characteristic combined with the high cost, testing restrictions, and the difficulty to control limited the use of selectively plating TAB leads.
Accordingly it would be desirable to have a low cost, controllable process for depositing the tin layer required on TAB semiconductor package outer leads.